1. You've hooked up before.
This is what separates the "just friends" from the "eh, we're just friends for now." At least once, you've had a few drinks, gazed longingly into each other's eyes, and went at it. There's a spark there. A physical attraction. Plus, you already love spending time with each other. Maybe it's happened more than once. Maybe more than 10 times. At that point, though, you just need to start dating.
2. They're your de facto date anyway.
Is there a new movie you want to see? You're dragging them along. New restaurant to try out? You've already made a reservation for you and your best friend. Wedding plus-one? Them. Friends all taking their SOs out? Best friend is coming too. You have all the same hobbies and interests, and when you're both single, you basically use each other for dates as it is.
3. Your family loves them.
Your family knows your best friend so well and they already approve (they even make sure to tell you that not-so-subtly). That's not enough on its own though. If you just went out with whoever your mom approved of, you'd be married to her old sorority sister's kid. But it's nice to know that if you did start dating your best friend, they'll just slide right into family gatherings and skip right past the awkward "meet my family" stage. It's a nice little bonus.
4. You've thought about dating them before, then they thought of dating you, but things never lined up right.
There was a time when you were into them, but they were with someone. Then they were into you, but maybe you found someone else, or moved away for college. Things just kept getting in the way, but you never dismissed the idea of being with them.
5. You've had relationships that didn't last because of how close you and your best friend are.
If you're looking for a major, giant, neon flashing sign that you belong together, this is as good as any. If you've ended serious relationships because you refused to give up your best friend in any capacity, that says a lot. Maybe it just says the right person hasn't come along yet, or maybe that right person is already there.
6. No one gets you like they do.
They always know how to cheer you up when you're down. They're the first one you call when you need help...and that includes current SOs. If your best friend trumps your serious boyfriend or girlfriend, that says a lot. You might not realize it consciously, but you'd rather be texting them exciting news than your own SO...that says a lot.
7. You've put off finding a significant other just because your best friend fulfills so many aspects of it already.
This one sounds cute, but in reality, it's probably not too healthy. If you feel like they're checking off a lot of the things on this list already and you don't feel the need to find someone else, then you should either commit to them or cool things down a bit so you can find a fulfilling relationship.
8. You've never tried to set them up with your friends.
Call it jealousy, but you just get a twinge of annoyance whenever you see them with someone else. You always find something wrong with everyone they're into. No one is ever good enough for them. Except you. You are clearly perfect for them. Duh.